A piezoelectric element is excellent in conversion efficiency and response from electric energy to mechanical energy. Therefore, a variety of piezoelectric actuators utilizing a piezoelectric effect of the piezoelectric element have been developed in recent years. The piezoelectric actuators have been applied to products such as a piezoelectric buzzer, an inkjet head of a printer, and an ultrasonic motor. Recently, applications of the piezoelectric actuators to those, such as a calendar indicating mechanism of a wrist watch, which have been strongly required to be miniaturized, have been studied.
FIG. 36 is a schematic illustration of a wrist watch having a calendar indicating mechanism using a piezoelectric actuator. As shown in the figure, the calendar indicating mechanism has a piezoelectric actuator A1, a rotor 1, an intermediate wheel 2, and a ring-shaped date indicator 3 on which a date and a day of the week are indicated.
The rotor 1 rotatably supported by a base plate (base) 4 so as to rotate about its own axis is driven to rotate by the piezoelectric actuator A1 in the direction indicated by the arrow Y shown in the figure. The rotor 1 engages with the intermediate wheel 2 rotatably supported by the base plate 4 so as to rotate about its own axis, and the intermediate wheel 2 engages with the date indicator 3. With this structure, as the rotor 1 driven by the piezoelectric actuator A1 rotates, the date indicator 3 rotates in the direction indicated by the arrow Z shown in the figure.
FIG. 37 is a plan view illustrating the structure of the foregoing piezoelectric actuator A1. As shown in the figure, the piezoelectric actuator A1 has a diaphragm 5 having a flat, strip-shaped piezoelectric element. The diaphragm 5 has an abutment member 6 at one vertex thereof where one of its long sides and one of its short sides intersect with each other, and the abutment member 6 has a top having a gently curved surface and protrudes from the piezoelectric element. Also, a support member 7 for supporting the diaphragm 5 on the base plate 4 is disposed in the vicinity of the center of the long side of the diaphragm 5 in a manner such that the support member 7 extends out from the diaphragm 5, and a spring member 8 is disposed so as to be opposite to the support member 7. The support member 7 is loosely fitted into a pin 7a disposed in a standing manner on the base plate 4 so that the diaphragm 5 can turn about the pin 7a. Also, the top of the spring member 8 is supported by a shaft 9 disposed on the base plate 4 in a standing manner. With this arrangement, the diaphragm 5 is pressed against the rotor 1 by an elastic force of the spring member 8, and the abutment member 6 abuts against the side surface of the rotor 1.
When an alternating drive signal is fed to the piezoelectric element, the diaphragm 5 vibrates in the direction indicated by the arrow X shown in the figure in a state where the abutment member 6 abuts against the rotor 1. This vibration is forwarded to the rotor 1 via the abutment member 6. The rotor 1 receiving this vibration as a drive force rotates in the arrow Y direction.
Meanwhile, in such a piezoelectric actuator A1, the amplitude of a vibration is on the order of micrometers or sub-micrometers. In order to continuously transmit such a fine vibration to the rotor 1, it is needed to press the abutment member 6 of the diaphragm 5 against the rotor 1 with an appropriate pressing force.
In order to achieve this, the support member 7 is fixed to the pin 7a with play (movable clearance), for example, in a manner such that the support member 7 has a perforation having a diameter slightly greater than that of the pin 7a and the pin 7a is inserted into this perforation, and the diaphragm 7 is arranged so as to turn about the pin 7a with an elastic force of the spring member 8.
This play (movable clearance) causes the following two problems.
First, the piezoelectric actuator A1 causes the other portion of the diaphragm 5, which is unnecessary for driving the rotor 1, to vibrate, thereby leading to a loss of energy and a reduced driving efficiency.
Second, since nonuniform contact of the abutment member 6 of the diaphragm 5 with the rotor 1 always causes its drive force for driving the rotor 1 to be unstable, the piezoelectric actuator A1 has an unstable driving characteristic.
In addition, since the diaphragm 5 itself vibrates, it is required to provide redundancy to wire lines or the like used for feeding a drive signal to the diaphragm 5, taking account of a change in length of the wiring route of the wire lines or the like. Furthermore, since the diaphragm 5 itself vibrates, connected portions of the wire lines are likely to fall off, thereby causing a problem of reduced reliability of the piezoelectric actuator A1.